The present invention relates generally to an electronic-controlled transmission and, more particularly, to a transmission in which the force applied to a shift rod is varied to obtain smooth and rapid shifting operation.
When operating a manually-operated gear transmission, a skilled driver applies a relatively large force to a shift lever during changes in low-speed stages and applies a smaller force during changes in high-speed stages. Therefore, no excessive load is exerted on the high speed transmission mechanism which is relatively weak structurally. However, automatic control of such a transmission is sometimes preferred.
A present electronic-controlled automatic transmission utilizes a hydraulic pressure circuit to control a clutch actuator for engaging and disengaging a clutch, and a shift actuator connected to a manually-operated gear transmission. The pressure circuit is controlled by the output of an electronic control device in response to signals dependent on the position of an accelerator pedal, the rotational speed of the engine, vehicle speed, etc. However, in the case where the transmission operating mechanism is uniformly driven by a common fluid pressure actuator, a relatively small applied force increases the time required for shifting operation in low-speed stages thereby impairing smooth acceleration of the vehicle. Conversely, a relatively large applied shifting force can possibly damage the transmission mechanism when shifting in high-speed stages.
For solving the aforementioned problems, Japanese Patent Application Laid-Open No. 63 (1988) - 167,156 discloses an electronic-controlled automatic transmission in which an electromagnetic valve inserted into a drain passage of a hydraulic actuator is closed to reduce applied shifting force and thereby prevent damage of a synchronous mechanism, and subsequently the electromagnetic valve is opened to increase applied force to attain rapid shifting operation. In the aforementioned electronic-controlled automatic transmission, however, the force applied by the hydraulic actuator is not independently controlled for shifting operation at each transmission stage. Consequently, smoothness of operation is impaired.
The object of the present invention, therefore, is to provide an electronic-controlled transmission which induces rapid shifting operation at all speed ranges while avoiding damage to the transmission operating mechanism.